The present invention relates to an air dryer assembly for use in a compressed air system for heavy vehicles, locomotives and/or auxiliary devices that use compressed air. Typical heavy vehicle compressed air systems include a compressor for compressing the air, an air dryer assembly for removing moisture and contaminants from the compressed air, a reservoir for storing the compressed air and valves and other pneumatic system components for distributing the compressed air. The present invention finds particular application in conjunction with heavy vehicles that use compressed air to selectively control application of vehicle brakes and will be described with particular reference thereto.
The compressor provides compressed air to a reservoir, the pressure being used to operate vehicle brakes and other air operated systems associated with the vehicle. Ambient air is typically drawn into a compressor inlet for compression during an air delivery, or charging, cycle. The air brake compressor is typically supplied with oil in order to lubricate bearings and other components of the compressor.
Typically, the air dryer assembly is installed downstream from the compressor for reducing the moisture content of the compressed air. The air dryer assembly delivers substantially dry compressed air for braking system components, thereby increasing the service life of braking system components such as valves and pneumatic brake actuators. Providing clean dry compressed air to brake system components reduces maintenance costs. In some air dryers, a desiccant is used to adsorb water vapor as the compressed air passes through the air dryer assembly during a charging cycle. Liquid water and water vapor is adsorbed onto the desiccant. After a period of charging during normal operation, the desiccant becomes saturated with water and must be regenerated. The desiccant is regenerated through a purging cycle, which comprises passing dry, high pressure air in a reverse direction through the desiccant.
Moisture and particulates may exist in the ambient air entering the compressor during a charging cycle and subsequently pass through the compressor to the air dryer assembly. In addition, lubricating oil may become entrained as an aerosol in the compressed air stream exiting the compressor. The air dryer efficiency is adversely affected by the particulates and the oil that can be deposited on the surface of the desiccant, reducing the desiccant's ability to adsorb water vapor.
It is desirable to reduce contamination of the desiccant with oil by employing filtering elements that effectively reduce oil in the compressed air stream. One type of filter is an oil removal filter, where liquid oil adheres to the filter during a charging cycle. The filter releases the collected oil into an oil sump. A second type of filter is an oil coalescing filter which coalesces oil from an aerosol form to a liquid form. A coalescing filter can be used in combination with an oil removal filter so that the liquid oil is captured after the oil aerosol is converted to a liquid in the coalescing filter.
Some prior art air dryer assemblies have a filter downstream of the desiccant during a charging cycle, which allows the oil and particulates to pass into the desiccant before being removed from the compressed air stream. While a downstream filter will reduce contamination in brake system components, the desiccant in the air dryer assembly will still be fouled by any oil and particulates in the compressed air stream as described above. This will make the desiccant less effective at removing moisture. The Puraguard® QC oil coalescing filter by Bendix Commercial Vehicle Systems LLC is an example of a separate filtering device used in a compressed air system downstream of the air dryer assembly. An example of an air dryer assembly having an oil filter within the assembly and downstream of the desiccant is the WABCO® Air System Protector with integrated coalescing filter by Wabco Holdings Inc.
While it is preferred to remove the oil and particulates prior to the compressed air reaching the desiccant, certain types of internal filters upstream from the desiccant during a charging cycle, as well as certain configurations and combinations of filters, may cause channeling of the compressed air flow in the desiccant. Channeling occurs when definite narrow air flow passages form in the desiccant and the compressed air flow is substantially confined to these narrow passages rather than flowing freely throughout the entire volume of the desiccant. This compressed air flow channeling results in only a portion of the desiccant defined by these channels being in contact with the moisture laden compressed air stream, thereby reducing the effectiveness of the air dryer assembly in removing the liquid water or water vapor. One cause of the deleterious channeling may be the configuration of the filter or filters upstream of the desiccant, which forces the compressed air into channels within the filter elements as the filter captures the oil and particulates. The channels formed in the filter element remain and confine the compressed air flow as the compressed air passes into the desiccant, thereby concentrating the compressed air flow into certain areas of the desiccant and reducing water vapor removal efficiency. Another cause of channeling may be liquid oil that passes through the filter and is adsorbed onto the desiccant itself. In this situation, the oil can form channels in the desiccant as the oil fouls the desiccant, thereby inhibiting the desiccant's moisture removal ability. Another cause of channeling may be a high velocity of compressed air longitudinally through the filter. Another cause of channeling may be inadequate pressure drop across the desiccant.
For the foregoing reasons, there is a need for a new and improved apparatus and method to remove oil and particulate contaminants from the compressed air stream, which addresses the above-referenced problems.